1. Field of the Invention
The present invention relates to controlling the feed of water treatment chemicals. More particularly, the present invention relates to the use of a voltammetric current measurement as a feedback signal for a controller that provides on-off or proportioning control of the introduction of chemicals for the treatment of water and wastewater.
2. Description of the Related Art
A wide variety of chemicals are added to industrial process, boiler, and cooling water for use as microbicides, corrosion inhibitors, scale inhibitors, etc. Likewise, chemicals are added to wastewater for similar purposes or as purifying agents, such as heavy-metal precipitants, flocculants, etc.
There are several reasons why it is desirable to control the level of these compounds in a water system. Adding too much treatment chemical (overfeeding) is wasteful and can prevent the treatment program from being cost-effective. Overfeeding can cause unacceptably high levels of treatment chemicals to appear in the discharge water which, in turn, may present environmental impact problems and may interfere with the operation of biological waste treatment facilities. In this manner overfeeding can cause an industrial facility to be in violation of its wastewater discharge permits.
Further, feeding the treatment chemicals at too low a rate, i.e., underfeeding, will cause the treatment program to be ineffective. In the case of microbicide use, there may not be enough chemical present to control the growth of microorganisms. In the case of wastewater treatment using a precipitant for the so-called "heavy" metals, i.e., those transition metals which are toxic and will cause environmental harm if discharged into rivers, lakes, or other natural water sources, underfeeding the precipitant will allow toxic levels of the heavy metals to be discharged. In this manner underfeeding can also cause an industrial facility to be in violation of its wastewater discharge permits. Thus, failure to control levels of water treatment chemicals can have obvious harmful consequences for the environment.
Two techniques for controlling the feed of a water treatment chemical have been proposed. In the first control technique, the treatment chemical is added until a small excess is detected, and then addition of the chemical is stopped as soon as possible to minimize overfeeding. Ideally, there would be no excess treatment chemical used. This technique is very similar to a titration. An example of a situation in which this technique is used involves the precipitation of lead (Pb++) or copper (Cu++) from a wastewater stream using sodium dimethyldithiocarbamate. The exact quantity of treatment chemical that is required by the stoichiometry of the metal-dithiocarbamate reaction would be used, since a significant level of excess dimethyldithiocarbamate ion is not needed to ensure complete removal of these metals from the wastewater. Since it is unnecessary to know the actual level of excess treatment chemical in the water, the method used to detect the treatment chemical need not be very precise or accurate; and a wide linear range will not be essential. However, the response time must be very fast to minimize overfeeding; and the method must be sensitive enough to give a detectable response to a small level of excess treatment chemical.
In the second proposed control technique, the water treatment chemical is added until a specific concentration level of the chemical exists in the water, and additional chemical is added as needed to maintain this level. An example of a situation in which this technique would be used is the addition of a microbicide to whitewater in a paper machine. A certain level of the microbicide (often 100 ppm or less) will be needed to inhibit the growth of microorganisms, and it will be necessary to maintain this level within certain limits. If the microbicide level drops too low, the population of microorganisms may begin to grow to levels that interfere with the operation of the paper machine. On the other hand, if the microbicide level is too high, the excessive chemical usage will waste money; it may cause problems (such as discoloration) in the manufacture of paper; and the chemical may appear in the wastewater from the paper mill and thus may cause wastewater discharge problems. The method used to measure the level of treatment chemical in the water must be sufficiently precise to accurately determine if the level of treatment chemical is within the desired range. While response time and sensitivity are also important, these characteristics generally will not be as critical for this situation as for the titration-type control technique described above. Sensitivity only needs to be high enough to make an accurate determination of the compound at the selected use level. Once the required level of treatment chemical is established in the system, changes in the level will be relatively slow, and the rapid response needed to halt the addition of the treatment chemical in the treatment method described above will not be needed. The design of control equipment and techniques that can be used to carry out both of these control procedures is an important object of this patent.
As shown in FIG. 1, control of any water treatment process, such as precipitation of heavy metals from wastewater, requires three fundamental components:
1. A chemical feed device 102 for which the speed (feed rate) can be electrically controlled will be necessary. This feed device will usually be a pump for the introduction of liquid treatment chemicals from, for example, treatment chemical bulk storage 104, but a screw feeder equipped with a variable-speed motor may be used to introduce solid treatment chemicals.
2. A sensor 106 and associated electronics 108 will be needed to detect the amount of treatment chemical that is in the system or is needed by the system. This sensor 106 will produce a feedback signal that is sent to a controller.
3. A controller 110 will be needed (a) to compare the feedback signal from the sensor 106 with a signal that would correspond to the desired level of treatment chemical and (b) to make adjustments in the speed of the chemical feed device 102 so that the level of treatment chemical detected in the water corresponds to the desired level.
These three components must be present in some form to maintain control over the level of water treatment chemicals used. It is especially true that, in the absence of feedback, effective control cannot be achieved. In some cases, a person may perform the function of one or more of the components. For example, in the simplest configuration possible, a person may take a water sample, analyze it chemically (the function of the sensor), calculate and weigh out the amount of treatment chemical needed (the function of the controller) and manually add the treatment chemical (the function of the chemical feed device). However, for many operations it would be preferable to perform these functions automatically. Automated control is less expensive than manual control in many instances, and a properly designed automated system should be able to control the levels of treatment chemicals more precisely and more reliably than human operators. Automatic controllers that can be used for this purpose will implement ON/OFF or proportional/integral/derivative (PID) control algorithms and are available from a number of manufacturers, such as Honeywell, Inc. of Minneapolis, Minn. and Fenwal, Inc. of Ashland, Mass. It is a primary object of this patent to render automatic control possible through the use of voltammetric sensors to provide the required feedback signal.
There are two fundamental approaches that can be used to generate a feedback signal for the controller. In the first of these two approaches, the sensor 106 responds directly to the concentration of treatment chemical present in the water and generates a feedback signal directly proportional to the concentration of treatment chemical. In other words, the feedback signal increases as the level of treatment chemical increases. An example of such an application might involve the use of glutaraldehyde or a dithiocarbamate salt to control the growth of microorganisms in the water. An appropriate sensor 106 would respond directly to the level of the microbicide in the water.
In the second approach, the sensor 106 may respond to a substance in the water with which the treatment chemical is intended to react rather than the level of treatment chemical. In this manner the sensor 106 would generate a feedback signal that is inversely proportional to the level of treatment chemical. In other words, the feedback signal would decrease as the level of treatment chemical increases. An example of such an application might involve the use of sodium dimethyldithiocarbamate to precipitate certain specific heavy metals from a waste stream. In a system that contains a very limited variety of metals, it would be possible to provide a feedback signal for each metal using anodic stripping voltammetry. An on-line device for making this type of measurement is available from Ionics, Inc. of Watertown, Mass.
Some situations will require the use of feedback signals that are directly proportional to the level of treatment chemical in the system. One example of this case would be the maintenance of a given level of microbicide as described above. Another example would involve the use of a dimethyldithiocarbamate salt to precipitate a variety of metal ions from a waste stream. In this instance it would be unnecessary to determine the level of each of the metal ions in the wastewater in order to adjust the amount of dithiocarbamate added; it would only be necessary to establish and maintain a predetermined level of excess dithiocarbamate in the waste stream. If there is a sufficient level of excess dithiocarbamate in the wastewater, then it may be assumed that all of the dissolved "heavy" metals have been precipitated. The determination of the dithiocarbamate concentration would be far simpler than the determination of the levels of all the heavy metals in the wastewater.
On the other hand, certain situations will require the use of a feedback signal that is indirectly related to the level of treatment chemical in the system. For situations that involve the removal of a toxic substance from a waste stream, this technique is desirable since the feedback signal not only controls the feed of treatment chemical, but also provides a direct, recordable measurement of the level of the toxic substance in the waste stream. Records of these measurements can be used to document compliance or noncompliance with the wastewater discharge permit of the facility. For example, the discharge permit of a wastewater treatment facility that uses sodium dimethyldithiocarbamate to precipitate heavy metals may have a limit on the level of dimethyldithiocarbamate ion that can be present in the final effluent water. A sensor 106 that responds directly to the level of excess dimethyldithiocarbamate ion in the waste stream can be used to generate a feedback signal to control the feed of a solution of ferrous ion, which reacts with and thus precipitates the excess dithiocarbamate ion. A recording of the level of dithiocarbamate ion detected, i.e., the feedback signal, will verify that the dithiocarbamate ion has been adequately removed from the waste stream. Unfortunately, not all toxic substances that must be removed from effluent wastewater can be determined conveniently by on-line analytical methods. It is another important object of this invention to provide a method for generating feedback signals that can be used for direct or indirect control of the feed of water treatment chemicals and for documenting compliance with the wastewater discharge permit.
To generate an effective feedback signal, the sensor 106 must perform a quantitative analysis of the process water or waste stream to control the level of treatment chemical desired. Many conventional laboratory techniques have been automated so that they may be used for on-line measurements. On-line equipment for colorimetric analyses is available from the Hach Co. of Loveland, Colo. Likewise, on-line equipment for turbidimetric analyses has been described in U.S. Pat. No. 4,923,599.
Electrochemical measurements are well suited as a basis for generating a feedback signal for several reasons:
(1) Many of the chemicals used in water and wastewater treatment may be determined using electrochemical techniques.
(2) The equipment needed for electrochemical measurements is inexpensive compared to the equipment needed for on-line colorimetric measurements or chromatographic (HPLC) measurements.
(3) Electrochemical sensors are fairly simple and are typically rugged and reliable. Unlike on-line colorimetric and turbidimetric measurements, which would require pumps to keep a portion of the process or waste stream flowing through the optical cells, electrochemical sensors do not have moving parts which have a high probability of failure.
(4) Electrochemical sensors are easier to maintain than colorimetric or turbidimetric which require time and labor intensive dismantling and cleaning. This feature is important because exposure to process or waste streams, especially those containing a high level of suspended solids, will rapidly contaminate the surface of any measurement device. If the electrochemical sensor is accessible, simple manual wiping may be sufficient for electrode maintenance. Inaccessible sensors require a different cleaning technique.
A proposed technique for making electrochemical measurements includes potentiometric methods, which involve the measurement of the voltage that develops on the surface of an electrode when it is immersed in a solution. The voltage is measured against a reference electrode, such as the silver/silver chloride (Ag/AgCl) couple or a saturated calomel electrode (SCE). Voltage measurement devices used for this technique must draw as little current through the electrodes as possible so that the electrode potentials will not be altered by the measurement. In other words, a very high-impedance measurement circuit must be used. In an ideal potentiometric measurement, no current should pass through the electrodes whatsoever. In practice, commonly-used voltage-measurement circuits are designed to draw less than one picoampere (1 pA or 10.sup.-12 ampere) through the electrodes. Maximum input currents in the low femtoampere (fA or 10.sup.-15 ampere) range can be achieved using currently-available electrometer amplifiers, such as the AD549L amplifier manufactured by Analog Devices, Inc. of Norwood, Mass.
However, using potentiometric measurements to generate a feedback signal in a control system have not provided satisfactory results. To begin with, the voltage that is measured in a potentiometric determination is directly proportional to the logarithm of the concentration of the substance that is being detected. This logarithmic relationship requires complicated electronic equipment to obtain a display of the measured concentration, e.g., %, ppm, etc. Hence, the logarithmic relationship obtained in potentiometric measurements lowers the accuracy and resolution of the concentration determination, and this limitation reduces the accuracy with which the concentration level can be controlled. In other words, the control system may not be able to detect and respond to changes in the concentration of the treatment chemical in the water unless those changes are large, i.e., changes by factors of 2-3 or more.
Further, the response time for potentiometric measurements can be very slow, especially for ion-selective electrodes used in solutions containing very low concentrations of analyte. This response time can be on the order of minutes, and a feedback signal with such a slow response time may not give the controller enough time to respond to a concentration change in the system, especially for a flow-through design. By the time such a sensor has responded to a sudden change in the demand for the treatment chemical, it could be too late for the control system to adjust the speed of the chemical feed device to maintain an adequate level of treatment chemical in the stream. During the time that the sensor is responding to the change in demand for treatment chemical, the wastewater that is discharged will be inadequately treated or will contain a large excess of treatment chemical. In either case, the discharge permit of the facility may be violated.
In addition, the performance of the extremely high-impedance measurement circuits required for potentiometric measurements can be severely degraded by the presence of moisture or chemical contamination, which are common in an industrial environment.
Finally, a mixed potential measurement, such as an oxidation-reduction (ORP) determination, is the net result of the influence of several factors, such as pH and the presence of oxidizing or reducing agents. There is no way to distinguish or resolve the different components that determine the measured potential.